Improved WAL API.

vfs/api.go

@@ -3,6 +3,7 @@ package vfs
 
 import (
 	"context"
+	"io"
 
 	"github.com/tetratelabs/wazero/api"
 )
@@ -154,17 +155,19 @@ type FileCheckpoint interface {
 	CheckpointStart() error
 }
 
-// FileSharedMemory extends File to possibly implement shared memory.
+// FileSharedMemory extends File to possibly implement
+// shared-memory for the WAL-index.
 // It's OK for SharedMemory to return nil.
 type FileSharedMemory interface {
 	File
 	SharedMemory() SharedMemory
 }
 
-// SharedMemory is a shared memory implementation.
+// SharedMemory is a shared-memory WAL-index implementation.
 // This cannot be externally implemented.
 type SharedMemory interface {
 	shmMap(context.Context, api.Module, int32, int32, bool) (uint32, error)
 	shmLock(int32, int32, _ShmFlag) error
 	shmUnmap(bool)
+	io.Closer
 }

vfs/file.go

@@ -118,13 +118,14 @@ func (vfsOS) OpenFilename(name *Filename, flags OpenFlag) (File, OpenFlag, error
 		syncDir: runtime.GOOS != "windows" &&
 			flags&(OPEN_CREATE) != 0 &&
 			flags&(OPEN_MAIN_JOURNAL|OPEN_SUPER_JOURNAL|OPEN_WAL) != 0,
+		shm: NewSharedMemory(name.String()+"-shm", flags),
 	}
 	return &file, flags, nil
 }
 
 type vfsFile struct {
 	*os.File
-	shm      vfsShm
+	shm      SharedMemory
 	lock     LockLevel
 	readOnly bool
 	keepWAL  bool
@@ -142,7 +143,9 @@ var (
 )
 
 func (f *vfsFile) Close() error {
-	f.shm.Close()
+	if f.shm != nil {
+		f.shm.Close()
+	}
 	return f.File.Close()
 }
 

vfs/shm.go

@@ -22,90 +22,107 @@ import (
 // [EXCLUSIVE locking mode]: https://sqlite.org/pragma.html#pragma_locking_mode
 const SupportsSharedMemory = true
 
-type vfsShm struct {
-	*os.File
-	regions []*util.MappedRegion
-}
-
 const (
 	_SHM_NLOCK = 8
 	_SHM_BASE  = 120
 	_SHM_DMS   = _SHM_BASE + _SHM_NLOCK
 )
 
-func (f *vfsFile) SharedMemory() SharedMemory { return f }
+func (f *vfsFile) SharedMemory() SharedMemory { return f.shm }
 
-func (f *vfsFile) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (uint32, error) {
+// NewSharedMemory returns a shared-memory WAL-index
+// backed a file with the given path.
+// It may return nil if shared-memory is not supported,
+// or not appropriate for the given flags.
+// Only [OPEN_MAIN_DB] databases support WAL mode.
+func NewSharedMemory(path string, flags OpenFlag) SharedMemory {
+	if flags&OPEN_MAIN_DB == 0 {
+		return nil
+	}
+	return &vfsShm{
+		path:     path,
+		readOnly: flags&OPEN_READONLY != 0,
+	}
+}
+
+type vfsShm struct {
+	*os.File
+	regions  []*util.MappedRegion
+	path     string
+	readOnly bool
+}
+
+func (s *vfsShm) shmMap(ctx context.Context, mod api.Module, id, size int32, extend bool) (uint32, error) {
 	// Ensure size is a multiple of the OS page size.
 	if int(size)&(unix.Getpagesize()-1) != 0 {
 		return 0, _IOERR_SHMMAP
 	}
 
-	if f.shm.File == nil {
+	if s.File == nil {
 		var flag int
-		if f.readOnly {
+		if s.readOnly {
 			flag = unix.O_RDONLY
 		} else {
 			flag = unix.O_RDWR
 		}
-		s, err := os.OpenFile(f.Name()+"-shm",
+		f, err := os.OpenFile(s.path,
 			flag|unix.O_CREAT|unix.O_NOFOLLOW, 0666)
 		if err != nil {
 			return 0, _CANTOPEN
 		}
-		f.shm.File = s
+		s.File = f
 	}
 
 	// Dead man's switch.
-	if lock, rc := osGetLock(f.shm.File, _SHM_DMS, 1); rc != _OK {
+	if lock, rc := osGetLock(s.File, _SHM_DMS, 1); rc != _OK {
 		return 0, _IOERR_LOCK
 	} else if lock == unix.F_WRLCK {
 		return 0, _BUSY
 	} else if lock == unix.F_UNLCK {
-		if f.readOnly {
+		if s.readOnly {
 			return 0, _READONLY_CANTINIT
 		}
-		if rc := osWriteLock(f.shm.File, _SHM_DMS, 1, 0); rc != _OK {
+		if rc := osWriteLock(s.File, _SHM_DMS, 1, 0); rc != _OK {
 			return 0, rc
 		}
-		if err := f.shm.Truncate(0); err != nil {
+		if err := s.Truncate(0); err != nil {
 			return 0, _IOERR_SHMOPEN
 		}
 	}
-	if rc := osReadLock(f.shm.File, _SHM_DMS, 1, 0); rc != _OK {
+	if rc := osReadLock(s.File, _SHM_DMS, 1, 0); rc != _OK {
 		return 0, rc
 	}
 
 	// Check if file is big enough.
-	s, err := f.shm.Seek(0, io.SeekEnd)
+	o, err := s.Seek(0, io.SeekEnd)
 	if err != nil {
 		return 0, _IOERR_SHMSIZE
 	}
-	if n := (int64(id) + 1) * int64(size); n > s {
+	if n := (int64(id) + 1) * int64(size); n > o {
 		if !extend {
 			return 0, nil
 		}
-		err := osAllocate(f.shm.File, n)
+		err := osAllocate(s.File, n)
 		if err != nil {
 			return 0, _IOERR_SHMSIZE
 		}
 	}
 
 	var prot int
-	if f.readOnly {
+	if s.readOnly {
 		prot = unix.PROT_READ
 	} else {
 		prot = unix.PROT_READ | unix.PROT_WRITE
 	}
-	r, err := util.MapRegion(ctx, mod, f.shm.File, int64(id)*int64(size), size, prot)
+	r, err := util.MapRegion(ctx, mod, s.File, int64(id)*int64(size), size, prot)
 	if err != nil {
 		return 0, err
 	}
-	f.shm.regions = append(f.shm.regions, r)
+	s.regions = append(s.regions, r)
 	return r.Ptr, nil
 }
 
-func (f *vfsFile) shmLock(offset, n int32, flags _ShmFlag) error {
+func (s *vfsShm) shmLock(offset, n int32, flags _ShmFlag) error {
 	// Argument check.
 	if n <= 0 || offset < 0 || offset+n > _SHM_NLOCK {
 		panic(util.AssertErr())
@@ -126,28 +143,28 @@ func (f *vfsFile) shmLock(offset, n int32, flags _ShmFlag) error {
 
 	switch {
 	case flags&_SHM_UNLOCK != 0:
-		return osUnlock(f.shm.File, _SHM_BASE+int64(offset), int64(n))
+		return osUnlock(s.File, _SHM_BASE+int64(offset), int64(n))
 	case flags&_SHM_SHARED != 0:
-		return osReadLock(f.shm.File, _SHM_BASE+int64(offset), int64(n), 0)
+		return osReadLock(s.File, _SHM_BASE+int64(offset), int64(n), 0)
 	case flags&_SHM_EXCLUSIVE != 0:
-		return osWriteLock(f.shm.File, _SHM_BASE+int64(offset), int64(n), 0)
+		return osWriteLock(s.File, _SHM_BASE+int64(offset), int64(n), 0)
 	default:
 		panic(util.AssertErr())
 	}
 }
 
-func (f *vfsFile) shmUnmap(delete bool) {
+func (s *vfsShm) shmUnmap(delete bool) {
 	// Unmap regions.
-	for _, r := range f.shm.regions {
+	for _, r := range s.regions {
 		r.Unmap()
 	}
-	clear(f.shm.regions)
-	f.shm.regions = f.shm.regions[:0]
+	clear(s.regions)
+	s.regions = s.regions[:0]
 
 	// Close the file.
-	if delete && f.shm.File != nil {
-		os.Remove(f.shm.Name())
+	if delete && s.File != nil {
+		os.Remove(s.Name())
 	}
-	f.shm.Close()
-	f.shm.File = nil
+	s.Close()
+	s.File = nil
 }

vfs/shm_other.go

@@ -12,6 +12,11 @@ package vfs
 // [EXCLUSIVE locking mode]: https://sqlite.org/pragma.html#pragma_locking_mode
 const SupportsSharedMemory = false
 
-type vfsShm struct{}
-
-func (vfsShm) Close() error { return nil }
+// NewSharedMemory returns a shared-memory WAL-index
+// backed a file with the given path.
+// It may return nil if shared-memory is not supported,
+// or not appropriate for the given flags.
+// Only [OPEN_MAIN_DB] databases support WAL mode.
+func NewSharedMemory(path string, flags OpenFlag) SharedMemory {
+	return nil
+}